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<div class="moz-cite-prefix">Some more findings I made while playing
with this series & GPUTop.<br>
Turns out the 2ms drift per second is due to timecounter. Adding
the delta this way :<br>
<br>
<a class="moz-txt-link-freetext" href="https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R607">https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R607</a><br>
<br>
Eliminates the drift. Timelines of perf i915 tracepoints & OA
reports now make a lot more sense.<br>
<br>
There is still the issue that reading the CPU clock & the RCS
timestamp is inherently not atomic. So there is a delta there.<br>
I think we should add a new i915 perf record type to express the
delta that we measure this way :<br>
<br>
<a class="moz-txt-link-freetext" href="https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R2475">https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R2475</a><br>
<br>
So that userspace knows there might be a global offset between the
2 times and is able to present it.<br>
Measurement on my KBL system were in the order of a few
microseconds (~30us).<br>
I guess we might be able to setup the correlation point better
(masking interruption?) to reduce the delta.<br>
<br>
Thanks,<br>
<br>
-<br>
Lionel<br>
<br>
<br>
On 07/12/17 00:57, Robert Bragg wrote:<br>
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<div class="gmail_quote">On Thu, Dec 7, 2017 at 12:48 AM,
Robert Bragg <span dir="ltr"><<a
href="mailto:robert@sixbynine.org" target="_blank"
moz-do-not-send="true">robert@sixbynine.org</a>></span>
wrote:<br>
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<div> at least from what I wrote back then it looks
like I was seeing a drift of a few milliseconds
per second on SKL. I vaguely recall it being much
worse given the frequency constants we had for
Haswell.<br>
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<div>Sorry I didn't actually re-read my own message properly
before referencing it :) Apparently the 2ms per second
drift was for Haswell, so presumably not quite so bad for
SKL. <br>
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<div>- Robert<br>
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